Dump Load / Diversion Load Intro

Since there is a lot of confusion about diversion loads, we will discuss what they are and why they are used.

A dump load is nothing more than a resistor (heating element) that is used for dumping electricity when it is not needed or to prevent your battery bank from being overcharged.

It could be a water heater or air heater or some other resistive load. Inductive loads (like a pump or television) aren’t usually the best idea for a dump load but they have been used successfully on occasion. Some AC heating elements make great dump loads.

The extra electricity is simply dumped or diverted into the diversion aka dump load.

There are three reasons why you might want to add a dump load to your off grid power system.

1. You have a wind turbine

2. You have a water turbine

3. You make more electricity than you consume

Do I need a diversion load for my system?

SOLAR MODULES
The short answer is maybe. Solar modules are a unique piece of equipment as they are very difficult to damage.

You can short circuit a solar module and nothing happens. No current flows.

You can open circuit a solar module and nothing happens. No current flows.

When your batteries are full and the solar modules are still producing power you have three good options:

Disconnect the solar modules from your battery bank

Short circuit the output of the solar modules

Dump the excess electricity into an air heater or water heater

Since solar electricity is expensive is does not make good sense to just waste the extra power as would happen if you disconnect or short circuit the modules. In this case a dump load (diversion load) makes much more sense. It is not necessary but likely a good idea.

WIND TURBINE
A wind turbine must always be connected to a load (that is what the batteries are). Unlike solar modules, a wind turbine will be damaged if allowed to open circuit. A wind turbine produces voltage according to the speed of the alternator. The faster the alternator spins, the higher the voltage. This is true of any alternator that doesn’t have a voltage regulator. If you remove the voltage regulator from a car alternator, it can easily produce 120 volts (10 times normal) or more.

In an off grid system, the battery bank will limit the voltage the wind turbine produces. When the turbine is connected to a 12 volt battery bank it will produce 12 volts (more like 13 or 14 volts) as it slowly raises the voltage of the battery bank until it is charged.

If the battery bank is full and the turbine is still producing power, that power must have a place to go.

As we have already discussed, a solar module can simply be disconnected, but a wind turbine cannot.

A wind turbine cannot be disconnected as the voltage will spike and damage components inside the turbine such as windings and rectifiers. Most wind turbines cannot be short circuited either as in a strong wind they will keep making power, heat up and possibly destroy the windings and/or diodes.

There are a few exceptions to this rule such as the Skystream by Southwest Windpower. The Skystream has a sine wave, grid tie inverter built in that allows the Skystream to connect with the grid without purchasing any controls. It is designed to check for grid power constantly and in the event of a power outage the turbine will apply an electronic brake. You can open circuit the turbine and it will simply shut down. Keep in mind the turbine still has a load which is the inverter. This turbine was built for the on grid folks but it works great as a battery charger as well. Check out this page to see how to use the Skystream to charge batteries.

WATER TURBINES
Like wind turbines, water turbines must always be connected to a load (or battery bank). A water turbine’s voltage is generally a little more stable than a wind turbine (as the water pressure doesn’t vary wildly like the wind) but can still spike to 3 or 4 times it’s usual output when allowed to open circuit. If your turbine is producing upwards of 60 volts in a 48 volt system, it could spike to 240 if allowed to open circuit. This voltage will likely not be en0ugh to damage the winding but can certainly destroy the rectifier or external charge controller.

AC COUPLING

AC coupling is becoming more common as folks who have their solar systems connected to the grid have become upset that their power goes out when the utility power goes out.

One solution to this issue is AC coupling. It is the act of adding a battery backup and battery inverter system to your grid tied system. When the power goes out, the solar array goes into the grid tie inverter, out of the grid tie inverter as DC and is fed to the output of the battery based inverter. After this electricity goes into the battery based inverter, it is converted to DC and charges the battery bank.

The danger of this system is there is nothing to stop the battery bank from overcharging. That is where the dump load becomes a necessity.

Thank you Jody Graham for your reply on the idea of exciting the Enphase M250 inverters, I understand every thing your saying except about the dump load. How do I build a dump load circuit that will sense when more power than is needed is being produced? You know, like if more power than needed will there be an increase in voltage in my 240vac output ? Since this is to be a 240vac dump load how is it designed ? Should a dump load be more than 4400 watts? Thank You William

Hi William,
What will happen when you are sending power from your Enphase back through your battery inverters, is the battery bank will charge. When the batteries are full you will need a dump load to prevent the batteries from being overcharged. If nothing is done the batteries will continue to absorb the electricity until they are destroyed. You do not have to worry about the Enphase making too much voltage as they are designed to only produce a little more than 240 volts to force the power backwards thru the grid or thru the battery inverter. Your dump load will need to be at the battery voltage as the batteries are the items in danger of overcharging. Go to our page about dump loads to see the different options for setting up a dump load for your particular system. And then go to our more advanced page on dump loads for solar systems. The dump load should be able to handle 4400 watts plus about 25% so that is not always maxed out and also allows for the extra power produced by the edge of cloud effect. Take a look at those two articles and let me know if I can clarify anything for you. Keep up the good work.Jody

Hello, I have a 4400 watt solar system with Enphase M250 Inverters on each of the 16 panels. The out put of these inverters is 240ac when tied to the grid. I’m told these Enpase Inverters can be excited by a 240vac inverter used for a back up system . Will I need a 240vac or higher dump load circuit for any extra electricity from my Solar Array so the Enphase inverters don’y over heat?

Hi,
Thank you for your comment. Your Enphase inverters need a 240 volt signal to make them operate properly. This is so they will shut down when the grid goes down. The Enphase inverters do not care where the 240 volt signal comes from as long as they get one and it is clean. You can connect the Enphase inverters’ output to the output of a battery based inverter made by Outback Power, Magnum Energy, Schneider electric or similar as long as the battery based inverter is 240 volts and a pure sine wave. When the sun shines, the Enphase will push power back through the battery based inverter and charge your batteries. A few things to remember. You need at least a 4400 watt battery based inverter system and preferably larger as you don’t want your battery based inverters to work at maximum power for hours on end. You will need a dump load or loads that can handle a little more than the 4400 watts as the Enphase may produce more than 4400 watts such as times during the edge of cloud effect. This process is known as AC coupling and is done quite often. We have a Sunny Boy 700 watt inverter that is tied into the output of our Outback Power inverters. We are off the grid and the distance between one of our 700 watt arrays is about 800 feet from our battery bank but only 30 feet from our home where the 120/240 volt power is available (we have a #4 wire from our main power system to our house that is about 800 feet long). We could have either ran a new wire 800 feet from our 700 watt array to the battery bank with a charge controller or simply wire the output of the Sunny Boy into our breaker panel and let the Sunny boy travel back to our Outback inverters and charge the batteries. Using AC coupling was the best idea for us and is in use all over the world. In many remote areas and underdeveloped countries it is common to setup one large battery bank with a large inverter for the community. Then as neighbors can afford it they add solar arrays with grid tie inverters that feed back into the main inverter and battery system. We will make a note to write an article on AC coupling in the near future. Thanks…Jody

Anytime you consume or dump electricity, heat is the byproduct. The more watts you dump into the heater the hotter it will become.

Most folks don’t know this but all electricity used (or consumed) creates heat. For example a 100 watt light creates 100 watts of heat. A 300 watt television creates 300 watts of heat. A 200 watt refrigerator creates 200 watts of heat. That is why you feel heat from a light bulb, the back of a TV and the back or underneath a fridge. For this reason it is silly to continually turn off your lights in the winter (to save electricity) if you heat your home with electricity. Every watt hour (or kilowatt hour) that your lights consume is one less watt hour (or kilowatt hour) your electric heater needs to produce.

So yes if you dump your excess power into a dump load such as a water heater element, resistor or air heater, it will create heat. Sometimes this is a good thing such as in the winter when you need the extra heat but if you have your air heating dump load in your home in the summer you will either need to use more air conditioning or live with the heat. Hope this answers your question and thanks so much for your comment. Jody

Greeting, wonderful website. I have a 500 Ah battery bank with only a 60 watts solar array and an MPPT charge controller just to maintain the bank fully charged since I only use it on very limited occasions. When I do use it a lot, I have a 55 amp power supply to quickly charge the bank. Question, should I be using a diversion (dump) load on a system like this? It seems that most days when the battery bank is fully charged my power analyzer shows very little power going into the batteries unless for some reason I apply a DC load to the batteries ( like my inverters). What happens to the extra power produced by the array that it is not shown as being sent to an already full battery bank?. Thank you

Thanks for your comment. I see no reason to have a dump load on your system. 60 watts of solar will not really make any usable amount of heat or hot water. Right now as the batteries fill up, the charge controller reduces the current going into the batteries by disconnecting the solar module(s). Solar modules can be open circuited (disconnected) with no issue. Water and wind turbines always need to be loaded by something, either batteries or a dump load. Solar does not.
Take Care…Jody